1. Field of the Invention
The invention relates to a metal halide lamp. The invention especially relates to a metal halide lamp of the short arc type which is suitable for a liquid crystal projector or the like, and which is operated using a direct current.
2. Description of Related Art
Liquid crystal projectors have recently become more and more important. A metal halide lamp of the short arc type is used for their light source. This light source consists of a metal halide lamp (hereinafter called only "lamp") and a concave reflector. It is formed by embedding one of the hermetically sealed portions of this lamp in the base opening of the concave reflector using a filler material in the state in which the lamp axis and the optical axis of the concave reflector agree with one another, or by similar processes.
The light emitted from the lamp is emitted directly or by reflection from the concave reflector onto an optical system such as a focussing lens and the like. The light which has passed through this optical system irradiates a liquid crystal cell. The image formed on the liquid crystal cell is projected via a projection lens onto a screen.
The metal halide lamp has hermetically sealed ends on both sides (is the so-called "double end type"). Its discharge space contains a cathode and an anode, mercury as the starting rare gas and various metal halides.
In this metal halide lamp, as a result of vaporization of the metal halides, a sufficient vapor pressure is obtained at a lower temperature than is the case when using metal elements. The radiant efficiency is higher than in a high pressure mercury lamp. In addition, by a suitable choice of the metals to be filled, outstanding color reproduction can be obtained. Therefore, the metal halide lamp is regarded as an optimum choice for use as a light source for a liquid crystal projector.
With consideration of the continuing miniaturization of liquid crystal projectors, and thus the miniaturization of the liquid crystal cell, there is a demand for a shorter distance between the electrodes of the lamp than in conventional cases in order to sufficiently focus the radiant light from the metal halide lamp on the liquid crystal cell.
Specifically, the distance between the electrodes in a conventional metal halide lamp was 3.0 mm to 5.0 mm. However, this requirement for miniaturization dictates that the distance between the electrodes be less than 3.0 mm.
With respect to this requirement for shortening of the distance between the electrodes, however, the follow disadvantages arise when only two electrodes are allowed to project into the discharge space:
In operation of the metal halide lamp, not all of the metal halides are completely vaporized, the metal halides being present in the discharge space partially as solids and partially as liquids. The metal halides in this solid state or liquid state collect during lamp operation on the coolest site (at the point with the lowest temperature) in the discharge space.
When the electrodes project into the discharge space, the temperature of the base points of these electrodes become, accordingly, lower until it reaches the coolest point. The metal halides in the solid state or the liquid state therefore collect in these base points. In doing so, the base point of the cathode represents the coolest part, since the anode generally has a higher temperature than the cathode.
The metal halides which have collected at this base point of the cathode penetrate the hermetically sealed portion into a very narrow gap which has formed between the cathode rod and the quartz glass. In this hermetically sealed portion, cracks occur due the effect of expansion and contraction and the like when the lamp is turned on and off.
With respect to the requirement for a shortening of the distance between the electrodes, a process can furthermore be theoretically imagined in which the entire discharge space is made smaller (i.e. a process for miniaturization while maintaining similarity). From the hermetically sealed portion, however, a certain size is necessary from a production standpoint. The hermetically sealed portions, in the case of a distance between the electrodes of less than 3.0 mm, are rather small. Production thereof entails major difficulties which could not be imagined in a conventional lamp.
Furthermore, the lamp can no longer be advantageously operated when, during operation, its tube wall load is not kept within a stipulated numerical range. Therefore, miniaturization of the discharge space is regarded as disadvantageous in conjunction with the tube wall load.